Re: [Regression] mm:slab/sheaves: severe performance regression in cross-CPU slab allocation

[Vlastimil Babka (SUSE)]

On 2/27/26 10:23, Ming Lei wrote:
> On Thu, Feb 26, 2026 at 07:02:11PM +0100, Vlastimil Babka (SUSE) wrote:
>> On 2/25/26 10:31, Ming Lei wrote:
>> > Hi Vlastimil,
>> > 
>> > On Wed, Feb 25, 2026 at 09:45:03AM +0100, Vlastimil Babka (SUSE) wrote:
>> >> On 2/24/26 21:27, Vlastimil Babka wrote:
>> >> > 
>> >> > It made sense to me not to refill sheaves when we can't reclaim, but I
>> >> > didn't anticipate this interaction with mempools. We could change them
>> >> > but there might be others using a similar pattern. Maybe it would be for
>> >> > the best to just drop that heuristic from __pcs_replace_empty_main()
>> >> > (but carefully as some deadlock avoidance depends on it, we might need
>> >> > to e.g. replace it with gfpflags_allow_spinning()). I'll send a patch
>> >> > tomorrow to test this theory, unless someone beats me to it (feel free to).
>> >> Could you try this then, please? Thanks!
>> > 
>> > Thanks for working on this issue!
>> > 
>> > Unfortunately the patch doesn't make a difference on IOPS in the perf test,
>> > follows the collected perf profile on linus tree(basically 7.0-rc1 with your patch):
>> 
>> what about this patch in addition to the previous one? Thanks.
> 
> With the two patches, IOPS increases to 22M from 13M, but still much less than
> 36M which is obtained in v6.19-rc5, and slab-sheave PR follows v6.19-rc5.

OK thanks! Maybe now we're approching the original theories about effective
caching capacity etc...

> Also alloc_slowpath can't be observed any more.
> 
> Follows perf profile with the two patches:

What's the full perf profile of v6.19-rc5 and full profile of the patched
7.0-rc2 then? Thanks.

Also contents of all the files under /sys/kernel/slab/$cache (forgot which
particular one it was) with CONFIG_SLUB_STATS=y would be great, thanks.

> 
> 
> -    8.30%     0.19%  io_uring         [kernel.kallsyms]               [k] mempool_alloc_noprof
>    - 8.11% mempool_alloc_noprof
>       - 7.64% kmem_cache_alloc_noprof
>          - 6.15% __pcs_replace_empty_main
>             - 5.96% refill_sheaf
>                + 5.95% refill_objects
> +    8.06%     0.44%  io_uring         [kernel.kallsyms]               [k] kmem_cache_alloc_noprof
> +    7.44%     0.00%  kublk            [ublk_drv]                      [k] 0xffffffffc140c71b
> +    6.63%     0.03%  kublk            [kernel.kallsyms]               [k] __io_run_local_work
> +    6.19%     0.05%  io_uring         [kernel.kallsyms]               [k] __pcs_replace_empty_main
> -    5.97%     0.01%  io_uring         [kernel.kallsyms]               [k] refill_sheaf
>    - 5.96% refill_sheaf
>       - 5.95% refill_objects
>          - 4.87% __refill_objects_any
>             - 4.76% __refill_objects_node
>                  0.72% __slab_free
>          - 1.00% allocate_slab
>             - 0.80% __alloc_frozen_pages_noprof
>                - 0.79% get_page_from_freelist
>                   + 0.72% post_alloc_hook
> +    5.96%     0.02%  io_uring         [kernel.kallsyms]               [k] refill_objects
> 
> 
> thanks,
> Ming
>